(1) Field of the Invention
The present invention relates to an optical disc apparatus and more particularly to tracking control
(2) Description of the Related Art
The following describes tracking control according to the state of the art, with reference to FIG. 7.
FIG. 7 illustrate various states in which a quadrant photodetector 116 receives light reflected from an optical disc without tracking control. FIG. 7 also illustrate a tracking error signal 11 (hereinafter, “TE signal”) generated responsive to the reflected light received by the quadrant photodetector 116 in each state. The reference potential illustrated in each FIG. 7 is the reference potential of a non-illustrated circuit for tracking control (hereinafter, “tracking control circuit”).
The tracking control circuit is designed to achieve tracking control in the manner described below. It is noted that the amplitude center of a TE signal corresponds to the center line of the track. Thus, under the condition where the TE signal amplitude center is not deviated from the potential used as a reference for tracking control, the tracking circuit operates to cause the TE signal to cross the reference potential at points corresponding to the center line of the track. As a result, it is ensured that the optical head is kept at a position over the center line of the track. In other words, the control circuit is designed to control a tracking actuator so as to minimize the difference between the TE signal amplitude center and the reference voltage 0. Ideally, the tracking actuator is driven so that the TE signal amplitude center becomes equal to the reference potential 0 illustrated in FIG. 7.
The TE signal is generated based on signals each indicative of light detected in a respective one of the light receiving areas of the quadrant photodetector 116. When the tracking control is OFF, the waveform of the TE signal 11 is, for example, sinusoidal as illustrated in FIG. 7. The sine wave represents changes in the amount of light received while the light beam traverses a plurality of tracks due to, for example, eccentric rotation of the optical disc. The TE signal deviates more and more from the reference potential 0 as the positional deviation of the light beam from the center line of the track increases. Similarly, the TE signal deviates less and less from the reference potential 0 as the positional deviation of the light beam from the center line of the track decreases.
Note that the dotted circles in FIG. 7 each represent abeam spot 10. FIG. 7A illustrates the state in which neither the quadrant photodetector 116 nor the beam spot 10 deviates from the center line of the track. FIGS. 7B and 7C illustrate the states in which the quadrant photodetector 116 and the beam spot 10 deviate from the center line of the track either inwardly or outwardly. In addition, the four quadrants of the quadrant photodetector 116 are divided into two light receiving areas along the direction of the tracks of the optical disc. In the figure, one of the light receiving areas is composed of quadrants A and D and the other composed of quadrants B and C. The TE signal is indicative of the difference between two signals output from the respective light receiving areas.
In the state illustrated in FIG. 7A, the quadrant photodetector 116 does not deviate from the center line of the track. Thus, the amplitude center of TE signal corresponds to the center line of the track and the TE signal is symmetrical about the reference potential 0. In such a case, the tracking control circuit drives the tracking actuator so as to minimize the deviation of the TE signal from the reference potential 0. As a result, the light beam is caused to follow the center line of the track.
On the other hand, in the states illustrated in FIGS. 7B and 7C, the quadrant photodetector 116 deviates from the center line of the track. Thus, the amplitude center of TE signal does not correspond to the center line of the track and the detected TE signal involves deviation from the reference potential 0. Yet, the conventional tracking control circuit still assumes that the TE signal amplitude center corresponds to the center line of the track and drives the tracking actuator to provide electrical offset to correct the asymmetrical TE signal to be symmetrical. The tracking actuator is then driven to minimize the deviation of the corrected TE signal from the reference voltage 0 with an intention to achieve precipice track following. Unfortunately, however, with this tracking control, the optical beam is caused to follow a radial position deviated from the center line of the track.
The documents listed below disclose conventional attempts to improve the accuracy of tracking control in the cases, as illustrated in FIGS. 7B and 7C, where the quadrant photodetector 116 and the beam spot deviates inwardly or outwardly from the center line of the track.
JP patent application publication No. 2000-20968 discloses a technique of easy and highly accurate servo balance adjustment for focusing control and tracking control during recording of an optical disc. According to the tracking control disclosed therein, the servo balance is adjusted so that the wobble components contained in a playback signal are kept equal or substantially equal to the minimum level.
JP patent application publication No. 2000-315327 discloses a technique of improving the accuracy of tracking control. According to the disclosure, if the position of a focusing lens is deviated at the initial state before tracking control, a TE signal detected at the initial state is corrected. More specifically, a TE signal is detected before tracking control, based on electric signals each indicative of the amount of light received by the photodetector and the balance of the TE signal amplitude is measured. The position of the focusing lens is shifted to provide symmetry in TE signal amplitude.